1. Field of the invention
The present invention relates to an SOI (silicon on insulator) substrate and a method for fabricating the same, which is made by attaching an active substrate constituted by a second semiconductor wafer to a base substrate constituted by a first semiconductor wafer.
2. Description of prior art
A conventional SOI substrate is obtained by attaching an active substrate constituted by a second semiconductor wafer to a base substrate constituted by a first semiconductor wafer, grinding or etching the unattached portion, then flat grinding the active substrate and processing the active substrate by polishing its upper portion to thin out the active substrate.
Accordingly, the flatness of an SOI substrate depends on the flatness of a base substrate semiconductor wafer. A semiconductor wafer with a high degree of flatness is hence requested to serve as the base substrate of an SOI substrate.
However, no matter what the flatness of a semiconductor wafer for the base substrate is, an SOI substrate as shown in FIG. 5a has to be processed by polishing the upper portion of the active substrate 8, and form a descending portion 8a as shown in FIG. 5b. As shown in FIG. 6, since the descending portion is provided with a small departing angle .alpha., the device engineering suffers from poor yield. The departing angle .alpha. is an angle between peripheral side surface of the active surface and the top surface of the active surface.